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Purpose And Requirements of Bright Annealing of Stainless Steel Pipe

Source:Iris Liang Time:2019-8-5 9:34:11

The purpose and requirements of the stainless steel bright annealing process:

(1) Elimination of work hardening to obtain satisfactory metallographic structure

The bright annealing furnace is mainly used for the heat treatment of the finished product of stainless steel under a protective atmosphere. When the performance requirements are different, the requirements for the metallographic structure after bright annealing are different, and the process of bright heat treatment is also different.

The typical heat treatment process for 300 series austenitic stainless steels is solution treatment. In the process of heating, the carbide is mixed into austenite, heated to 1050-1150 degrees Celsius, and kept for a period of time, so that the carbides are all dissolved in austenite, and then rapidly cooled to below 350 degrees Celsius to obtain a supersaturated solid solution, that is, a uniform single item. Austenitic organization. The key to this heat treatment process is rapid cooling, requiring a cooling rate of 55 ° C / s, and rapid re-precipitation temperature zone (550-850 ° C) after solid solution. The holding time should be as short as possible, otherwise the crystal grains will be coarse and affect the surface finish. SEKO Machinery's intelligent heat preservation insulation stainless steel tube bright annealing furnace has been very mature in the field of steel tube annealing technology after strict design and rigorous repeated debugging.
The 400 series ferritic stainless steel has a relatively low heating temperature (about 900 °C), and is mostly annealed. It can also be treated by sectional quenching and tempering.

From the above, it can be seen that the 300 series and the 400 series stainless steel differ greatly in heat treatment, and in order to obtain a qualified metallographic structure, there is a great room for adjustment of the cooling equipment for the bright annealing. Therefore, modern advanced bright annealing furnaces often use strong convection cooling in their cooling sections.

(2) Obtaining an oxidized bright surface

Bright annealing is the heat treatment of stainless steel pipes under the protective atmosphere of hydrogen and nitrogen. In order to meet the requirements of the bright tube, the protective atmosphere in the furnace must be strictly controlled to avoid oxidation. How does oxidation occur under a protective atmosphere of hydrogen and nitrogen?

The main alloy components of stainless steel are Fe, Cr, Ni, Mn, Ti, Si and the like. The oxidation of Fe and Ni is not a major problem in the annealing temperature range. However, the oxidation interval of CR, Mn, Si, and Ti is just within the heating temperature range. The oxidation of these alloying elements officially affects the surface brightness of steel pipes. In particular, the oxidation of chromium causes the surface of the steel tube to be dechromized, which reduces the corrosion resistance of the stainless steel. When the Cr content is between 17% and 18% and the Ti content is 0.5%, the hydrogen dew point must be lower than -60 °C to avoid the oxidation of Cr and Ti in the heating range of 800-1150 degrees Celsius.

In addition, how to maintain the purity of the gas in the furnace is also a key issue. In this regard, the induction furnace is well sealed.

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Iris Liang
SEKO Machinery & Technology Co., Ltd
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